public class lce {
    /*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val,Node _left,Node _right) {
        val = _val;
        left = _left;
        right = _right;
    }
};
*/
    class Solution {
        Node pre,head;
        public Node treeToDoublyList(Node root) {
            if(root == null){
                return null;
            }
            dfs(root);
            head.left = pre;
            pre.right = head;
            return head;
        }
        void dfs(Node cur){
            if(cur==null){
                return;
            }
            dfs(cur.left);
            if(pre != null){
                pre.right = cur;
            }
            else{
                head = cur;
            }
            cur.left = pre;
            pre = cur;
            dfs(cur.right);
        }
    }
}
